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In this project a study was conducted on two types of the composite materials carbon fibre 5ply laminate and carbon fibre sandwich structure.
The methodology of the project was categorised into manufacture of composite and their testing. In order to proceed the manufacture stage, the research was conducted and gained knowledge enabled to produce carbon fibre 5ply laminate and carbon fibre honeycomb structure.
These procedures were; the three-point-bend test, tensile test, drop impact test and charpy test. The all tests were performed on standard university equipment. After testing stage was completed the broad data was analysing and discussed.
Aim of the project
The aim of the project is present the way in which composites are manufactured, to conduct their testing and to gain understanding about their properties need to design and manufacture specific component. There is a large number of composite materials but due to limited time scale only carbon fiber composites and honeycomb sandwich will be studied in this project. Student will learn manufacturing techniques and test methods and will be able to make relevant observations and conclusion. The four types of test will be perform: tensile test, three-point-bend test, drop test and charpy test. From the analysis of the results material properties will be determined and discussed.
Both the drop test and three-point-bent test will be perform on two type of specimens made by student. The charpy test and the tensile test will involve only the single laminate sample due to structural requirements. For testing purposes the specimens were produced in variety of sizes and shapes as follows: for tensile test the dog bone shape, three-point-bend test the rectangular samples, for drop test the square specimens with drilled holes in each corner and for charpy test small rectangular sample with notch in the middle.
Summarizing, the aim of this project is to learn manufacturing the laminated composite materials, conduct a number of tests and analysing the obtained results.
To study the theory of composite materials in order to select the material and its structure to be used in the project.
To conduct the research of manufacturing methods of carbon fiber composites and honey comb structure
To review the theory of mechanical properties used for analysis and discussion obtained results
To produce specimens to be used for testing
-carbon fiber: dog bone shape, reqtangular, square.
-sandwich structure: rectangular and square.
To ensure the project is carry out in compliance with safety regulations
To conduct testing using drop test, tensil test, charpy test
To obtain results from tested specimens
To analyse results, discuss and conclude
What are Composites
The composites are materials comprised of two or more distinct constituents physically bonded on a macroscopic scale. The constituents have different physical properties from each other and form composite phase which has considerable different properties form its constituents. Other materials such as metals are also combined on a macroscopic scale, however the resulting material is macroscopically homogenous where constituents cannot be distinguish by naked eye.  The fundamental feature of composites is that they exhibit the best properties of their constituents.
Generally, the composites are light fibers with high-strength and high-modulus acting as a reinforcement in surrounding them matrix which contribute to the increase in the volume of the structure.
The fibers can take form of the single fiber or its multiple twist as yarn or tow,
continuous fibers, short fibers, whiskers, platelets
Regardless of type of fiber used, there is always a noticeable boundary between them, therefore either fibers and matrix retain their own physical and chemical properties. Despite that, they create unique characteristics that they cannot obtain when acting alone.
The fibers act as a load carrier, whereas surranding matrix keeps them in the required location and orientation, acts as a load transfer body between them, and protects them from environmental and mechanical damages.
The fibers employed in the industry for a commercial scale are glass-, carbon fiber and Kevelar which can be used in form of continuous or chopped fibers. The common matrix materials are polymer, a metal, or a ceramix.
There is a number of combinations and arrangements of fibers and matrix available. The most widely used form is laminate made of layers of fibers and matrix in order to obtain required thickness and desired properties.
The group of composites with outstanding characteristics such as strength and stiffness is defined with term advanced composites to highlight high-performance properties of the material.
3.2 History and Use
The idea of composites derive not from human but it originates in nature. Excellent examples are wood that consists of fibrous cellulose in a matrix of lignin and mammalian bone where collagen fibrils embedded in protein-calcium phosphate matrix form number of bone layers.
First evidence of using composites by human was described in the Old Testament in the book of Exodus, where the Israelites were adding straw to clay bricks in reinforcing purposes. It is also know that plant fibers were used in producing pottery by natives of South and Central America.
Another application with use of composites found Mongolian in arcs production. The stretched part is a combination of wood and cow tendons bonded together.
Japanese swords and sabers blades are made of steel and soft iron where the steel layers are formed into U shape into which soft iron is placed. It allows to obtain excellent resistance to flexure and impact.
First use of carbon fiber was about 100 years ago as filaments in electric lamp. However those fibers were fairly weak and with small quantity of reinforcement in comparison to today's high-performance carbon fibers.
The development of carbon fibers took place in 1960s and early 1970s simultaneously to development of following resins: 1969 (phenolics) and 1973 (epoxies) and many other thermosetting resins such as polyimides, phenolics, vinyl esters, furanes, silicones, polyurethanes and urethane acrylates.
3.3. Advantages and Disadvantages
Undoubtedly, the significant advantage of composite is high-strength-to weight and modulus-to-weight ratios. Apart from that, there is number of following advantages:
Lightness as result of high ratios mentioned above
Tailorable strength and stiffness properties in order to locate them in the load direction
Redundant load paths (fiber to fiber)
Non-corrosive properties resulting in longer life of component
Reduction of manufacturing cost due to lower number of parts required
Ease with forming desirable component structures such as aerodynamic, hydrodynamic
Increased (or decreased) thermal or electrical conductivity
Availability of variety of manufacturing techniques
Relatively high cost of raw material and fabrication
Structural weakness of the matrix and its low durability
Environmental problem as large number of matrix cannot be degraded
Difficulty with analysis due to hidden defects
3.4. Application of Composites
Composite materials are widely utilized in automotive, appliance, corrosion, electrical aerospace, marine, architectural industry and are used in production of some sport equipment such as tennis rackets, skis, golf sticks etc.
Figure 3.1 represents the consumption of composites in different industries.
Figure 3.1. Estimated 2004 composite consumption 
Since steering of aircrafts is highly dependent on their weight, high specific strength and stiffness became incredibly valuable for the entire aerospace industry.
Hence, due to significant contribution of aerospace structure to development of composites, the large part is devoted to this sector.
Application of composites provides aircrafts with smooth surface due to lack of rivets and sharp transitions that occur in metallic structures what result in elimination of drag.
The aircraft parts made of composites for instance flaps, wing skins, and various control surfaces are used in in following fighter models: F-14, F-15, F-16, â€¦., F-22.
The figure 3.2. depicts the use of composite material in military aircrafts since they were launched in early 1970's.
Figure 3.2. Composite structual application in military aircraft.
Due to decrease of material cost and technology development composite materials became much more popular in entire aerospace industry.
A 1994 NASA announced excellent performance of composite components in commercial aircraft which resulted in increase of their use in aircraft structure in small business-type aircraft as well as in large, commercial-transport aircraft.
Originally, composited were used only in small structures which carried only relatively light load, but today they are widely used also in large, major structures such as the wings and fuselage.
The excellent example represents the Cirrus SR-22 single-engine, depicted in figure3.3 ,with space for four passengers. The fuselage and the wings of this aircraft are entirely made of composites.
Figure 3.3. Cirrus SR-22 single-engine aircraft. 
Composites found great application in aerospace industry mainly due to their lightness which allows to add an extra weight of critical systems such as aeroplane parachute and contributes to savings of fuel.
The Boing 777 shown in Figure 3.4 consists of about 10% of composite materials, mainly graphite and epoxy. There also other commercial aircrafts such as Boing 787 and Airbus 350 which fuselage and wings are made of composites.
Figure 3.4. application of composites in the Boing 777. 
Due to high stiffness-to-weight ratio, low thermal expansion coefficient, and good vibration-damping characteristics composite materials are used greatly also by NASA in structures such as such as graphite/epoxy cargo bay doors and experimental graphite/epoxy solid rocket-booster motor cases and space station. Composites are extremely valuable material for aerospace due to weight save.
Moreover, composite found also application in I-beams, channel sections, and other structural elements used in civil infrastructure because of corrosion resistance and thermal insulation which is a great advantage over the metal parts.
The high strength-to-weight ratio, high stiffness-to-weight ratio, excellent vibration damping, and fatigue resistance properties are employed for manufacture structures such as wind turbines, in automotive vehicles, the bodies of transportation vehicles and the bridge decks. Weight savings on specific components can exceed 70% compared with steel elements.
A related application is wrapping concrete-steel bridge columns with composite tape in order to decrease impact of earthquakes.
3.5. Carbon and Graphite Fibers
Carbon/graphite fibers are major reinforcement with outstanding properties used in production high-performance polymer-matrix composites.
In the graphite structure, the carbon atoms form the hexagonal layers with a very dense packing in the layer planes.
The presence of the high-strength bond between carbon atoms in the layer plane contributes to an immensely high modulus, whereas the weak van der Waals -type bond between the neighbouring layers result in a lower modulus in that direction.
The presence ofÂ genuine graphite crystal structure in the fibers is impossible, hence term "graphite fibers" refers to fibers with a carbon contest of more than 99%. Fibers where a carbon contest accounts for 80-95% are called "carbon fibers". Carbon contest depends on the heat treatment temperature (process described below).
Nowadays, in fabrication of carbon fibers the commonly used technique is decomposition of organic precursors such as polyacrylonitrile (PAN), pitch, and rayon.
The precursor material that undergoes pyrolysis should to meet following criteria:
Adequate strength and properties allowing for "holding the fibers together" during entire conversation process.
Non-melting properties of the precursor. The precursor may be naturally infusible or stabilized.
Not complete volatilization of the precursor. The remain carbon yield of precursor after conversation process contributes to forming the graphite structure of the fiber, enhancing its mechanical properties and economic value.
The reasonably low cost of precursor material.
There are five stages of the process converting PAN into carbon fibers:
Spinning the PAN into a precursor fiber.
Stretching the precursor.
Stabilization by holding the prestretched polymer under tension at a temperature of 205-240 Â°C for up to 24h in an oxidizing atmosphere (air).
Carbonization at approximately 1500Â°C in an inert atmosphere. Carbonization is the process of pyrolizing stabilized Pan fibers to drive out most (if not all) noncarbon elements of the precursor fibers until they are essentially transformed into carbon fibers. It is during this stage that the high mechanical properties found in most commercially available carbon fibers are developed.
Graphitization at approximately 3000Â°C in inert atmosphere. Graphitization heat treatments are carried out at temperatures in excess of 1800Â°C in order to improve the tensile modulus of elasticity of the fiber by improving the crystallite within each individual fiber.
(drive out non carbon elements) Inter atmosphere
(improve crystallite structure)
Figure 3.5. Process of Converting PAN precursor fibers to carbon fibers.
There are different advantages and disadvantages of carbon fibers produced from each precursor. The PAN-based carbon fibers are leading type on the market due to good properties and relative low cost. They find great application in military aircraft, missiles, and spacecraft structures.
The carbon fibers with higher stiffness and thermal conductivities are obtained from pitch, therefore they are widely used in satellite structures and thermal-management applications eg. space radiators and electronic enclosures.
Due to low thermal conductivity of rayon-based carbon fibers, they are used as insulating applications of for example rocket nozzels, missile re-entry vehicle nosecones, and heat shields.
Carbon fibres properties can be adjust with ease during the manufacturing process. Constantly developing technology allows to obtain greater ranger of fiber properties and enable to expand their applications.
There is a variety of forms of carbon fibers such as: continuous, chopper, woven fabric, or mat. Within continuous-graphite fibers the most popular are tows, yarns, rovings, and tape. A tow is built up of filaments in straight-laid bundle which number vary from 400-10,000 or even 160,000. A yarn is a twisted tow. Roving is a continuous, parallel bundled group of fibres which are not twisted (or twisted very little). A tape is composed of number of tows or yarns (e.g. 300) beside each other and joined together to run longitudinally.
3.6. Fabrication of Composite
The manufacture of composite material is strictly connected to the sort of application the material is used for. The fabrication process of the structural element or even the complete structure begins at the stage of fabrication of building material. Hence, it is crucial to choose adequate manufacturing process as well as fiber and matrix type.
3.6.1 Bag Molding Process
The Bag molding process is a method where simply saying preimpregnated or freshly impregnated with wet resin laminae are laid up in a mold, covered with flexible diaphragm or bag, and cured with heat and pressure. After curing, the material adopts mold shape with cohesive structure. However, the process requires more materials that are essential for manufacturing:
Release agents - prevent adhesion of composite material to the mold and facilitates its removal.
Peel piles - protect the mold surface from impurities.
Release films - separate bleeder or breather plies from the composite laminate. It could be porous to enable resin to flow through the film.
Bleeder and breather plies - porous fabrics that absorb surplus resin. Moreover, breather plies acts as a pathway leading away air and volatiles from the composite material.
Bagging films - separate and protect composite material from autoclave environment.
In this process laying up, bagging and worker's skills are essential factors and considerably affect the quality of component. The size of the component is limited to the size of curing equipment, particularly to the size of oven or autoclave.
In term of pressure and applied heat the bag molding process can be divided into pressure bag, vacuum bag and autoclave.
In pressure-bag molding, the laminate is subjected to pressure above atmospheric and oven heat inside the closed mold.
In vacuum-bag molding, all impurities in form air and volatiles between the bag and laminate are eliminated by a vacuum pump. The atmospheric pressure is applied during oven curing.
Autoclave processing is an extension of the vacuum-bag method. Curing of laminate in autoclave is often under pressure, the composite is laid up and sealed in a vacuum bag. The process starts with raising the temperature in the autoclave chamber and general pressure which increases action of mechanical forces on the lay-up. The efficiency of transport of volatiles to the vacuum ports is improved, hence there is increase in wetting and flow of the resin. Due to removal of major quantity of air and volatiles the voids content considerably declines.
Vacuum-bag and autoclave are very common techniques in producing bag-molded components mainly due to low tooling cost and low exploitation of curing apparatus (oven and autoclave). The process is not limited to few mold shapes, therefore it may be used for variety of parts. However, the tooling can be used only for a specific part for which it is designed. It is also exposed to high pressure what causes its wearing and results in increasing in cost.
3.6.2. Hand Lay-up Technique
The type of fibers used in this process vary from woven fiber mat to chopped strand mat for hand lay-up method and chopped fibers for spray lay-up. The process is used for development work, prototype fabrication and manufacturing large components in small quantities. There are two types of mold: cavity (female) and a positive (male) mold.
After applying a release agent a mold is coated with gel that ensure smooth surface that in case manufacturing boat hulls or aircraft exterior parts is extremely important. Typical release agents are: wax, polyvinyl alcohol (PVA), fluorocarbons, silicones, release papers and release films, and liquid internal releases. The gel coat is usually a polyester, mineral-filled, pigmented layer or coating and it is an outer surface of the laminate. When thermosetting polymer resin and the fibers are laid up, a roller is used for consolidation the layers. Next, adequate temperature is applied to complete curing process.
The development of "prepreg tape " contributed to improvement of entire manufacturing process of composite materials. A tape with fibers precoated with polymer resin makes there is no longer need of mixing the resin constituents in the right proportions and paying attention to combine properly the resin with fibers. Using of thermosetting resin allows to store the tape at the room temperature until it is melted during final use. The big number of prepreg tape is produced in the process called hot-melt. The method with use of prepreg tape employ laying-up the tape orientated adequately on a mold following curing the laminate at required temperature and pressure.
Pultrusion is an automated manufacturing process for producing continuous lengths of reinforced polymer structural shapes with constant cross-section profiles. The name of the process refers to the way in which the composite is fabricated, even though there is a common link with extrusion the part is pulled out through a resin and heated die instead of forced out by pressure. There is number of profiles produced in this process by the use of different die such as rods, tubes and variety of other structural shapes.
In this process continuous rovings filaments, fibers bundles or continuous glass mats are pulled through a liquid resin which saturates the the glass reinforcement. Next the material is pulled through appropriate die using continuous pulling device, the surplus resin and air are removed. The mass of fabricated composite adopts to the shape of the die and in two temperature zones the product is structurally shaped.
Materials manufactured in pultrusion process benefits in high strength to weigh ratio, corrosion and heat resistance, dielectric properties as well as dimensional stability. Apart from variety of thermosetting polymer resin such as polyester,Â polyurethane,Â vinylester,Â epoxy, thermoplastic matrices can also be used in this technique.
Definition of sandwich structure
A sandwich structure (honeycomb) is two thin facings or skins are assembled by bonding or welding on a lighter core that is used to separate them two. The core material has usually low strength properties, however its higher thickness provides the structure with bending stiffness with overall low density. The characteristics of this honeycomb composite are very light weight, high flexural rigidity and excellent thermal insulation properties. However, certain types of core are combustive and they are more prone to undergo buckling. There is also a risk of trapping a liquid in a cellular structure which is difficult to localize and removal.
["Composite Material Design and Applications", second edition, Daniel Gay, Suong V.Hoa page 53]
The core of honeycomb sandwich as well as facing can be build of variety of materials and configurations. Within honeycomb core there are most common:
4.2.1. Hexagonal Core
The Hexagonal Core configuration represents the most basic and most popular structure. It can be made of all metallic as well as non-metallic materials. The size of the cell influences the cost of the structure. Smaller cells are more expensive but contribute to better bonding area. However, bigger and cheaper cells may lead to skin surface damage by 'dimpling effect'.
Figure 4.1. Hexagonal Core Configuration
The OX-Core is hexagonal configuration with the expansion of cells in ''W'' direction and little curve in "L" direction. The cells are forming rectangular shape structure which, in comparison with hexagonal configuration, provide higher shear strength in "W" direction and smaller in "L" direction.
Moreover, the hexagonal cells provide minimum density for a certain amount of material while producing the rectangular shape in "W" direction is more convenient during the manufacturing process.
Figure 4.2. OX-Core Configuration
The Flex-Core structure represents 'wave' shape configuration with non-buckling properties of the cell walls where anticlastic curvature is considerably minimized. Flexi-Core with tight radii of the curvatures is easy to manufacture and results in increasing of shear properties when compared to hexagonal configuration with the same density. It is available mainly in aluminium, NomexÂ®, and fiberglass substrates.
Figure 4.3. Flex-Core Configuration
This configuration provides the sizable aluminium cells of specific shape. The characteristic of Double-Flex is outstanding formability and high specific compression properties. It is the most formable cell configuration.
Figure 4.4. Double-Flex Configuration
This hexagonal configuration is reinforced with a sheet of substrate material that is placed horizontally in the "L" direction in order to improve mechanical properties. The Reinforced Hexagonal is able to carry high load due to its heavy density, therefore is ideal for attachment points etc.
Figure 4.5. Reinforced Hexagonal Configuration
The Tube-Core structure is designed to absorb the energy without the loss of crush strength that arises at the unsupported edges of standard honeycomb. This configuration consists of
alternate sheets of flat aluminium foil and corrugated aluminum foil wrapped around a mandrel and adhesively bonded. The diameter can vary from 1/2 inch to 30 inches and lengths from 1/2 inch to 36 inches.
Figure 4.6. Tube Core Configuration
Manufacturing of honeycomb core
The honeycomb core is mainly manufactured by expansion, however in order to obtain high density structure the corrugate method may be employed.
The manufacturing of honeycomb by expansion method initiates with the stacking of sheets of the substrate material on which adhesive node lines have been printed.
The adhesive lines are then cured to form a HOBE block, where HOBE represents Honeycomb Before Expansion.
After curing is completed, the HOBE block could be expanded in order to provide an expanded block. Slices of the expanded block could be subsequently cut to the preferred T dimension.
HOBE slices may also be cut from the HOBE block to the adequate T dimension and then expanded. Slices can be expanded to regular hexagons, underexpanded to 6-sided diamonds, and overexpanded to nearly rectangular cells.
The expanded sheets are cut to the required L dimension (ribbon direction) and W dimension (transverse to the ribbon).
The expansion process demands considerably high inter-sheet bond strenght
The expansion process is depicted in figure below:
4.3.2. Corrugated Process
The purpose of producing honeycomb by corrugate method is to obtain high density product.
In this process there are following steps: the adhesive is applied to the corrugated nodes, then the corrugated sheets are stacked into blocks. Next, the node adhesive cured, and in the end the sheets are trimmed to the desired core thickness.
Honeycomb Sandwich Production Methods
There are there general methods for manufacture of honeycomb sandwich:
Heated Press, used for the fabrication of flat board or simple preformed panels.
Vacuum Bag Processing, used for curved and complex form panels.
Matched Mould Processing, used for batch manufacture of finished panels.
In this method it is required panel to be assembled prior to curing as a single shot process. The facing skin is be a metallic material as well as a prepreg which could also be pre-cured by using a press, followed by bonding with a film adhesive layer. Further bonding does not require advanced equipment and simple tooling can be used to complete the assembly.
Vacuum Bag Processing
In the vacuum bag method the component is required again to be assembled for cure as a single shot process.
The vacuum is obligatory in order to achieve consolidation. The curing takes place in an oven or in autoclave where possible higher pressure can be applied. In this method as a facing skins can be used materials such as the prepreg, preformed composite or metallic.
Match Mould Processing
The best advantage of this process is that it allows to obtain high levels of tolerance and surface finish honeycomb composites. The method is perfect for single shot cure process where the heat and pressure cure cycle are applied by the heated tools with external mechanical pressure or non heated tools placed in a press or oven. The room temperature curing adhesive cold bonding may also be used in case the produced part is too big for available equipment or when the equipment is simply unavailable.
The strength and stiffness of honeycomb is proportional to the density which is illustrated in figure:
When the load is applied at the free end of a sandwich panel, the bending moment is created with its maximum at the fixed end. A shear force occurs along the length of the entire structure. The bottom skin is in compression and the top skin in tension and the core transfers shear forces between them in order to enable entire panel to act as homogenous structure.
Fugure 4.9 Load transfer in a honeycomb
The deflection of a honeycomb consists of bending and shear components. The bending deflection is related to tensile and compressive forces of the facing, the shear deflection refers to shear modulus of the core. However, when the span of the panel is large compared to its thickness, the shear deflection will be negligible.
The deflection of the sandwich panel can be schematically described as:
Total Deflection = Bending Deflection + Shear Deflection.
Figure 4.10 Deflection of Honeycomb
The additional parameter that affects panel deflection (as well as the skin stress) is also material thickness.
There are two major types of compressive strength: the stabilized- and the bare. Fundamentally, the stabilized compressive strength is the ultimate compressive strength of the honeycomb when loaded in the T direction.
The bare compressive strength represents the ultimate compressive strength of the core when loaded in the T direction without stabilization of the cell edges. The value is normally used for an acceptance criteria.
Figure 4.11. Honeycomb Compressive stress-Strain Curve.
The crush strength is the average load applied per unit cross-sectional area that causes a crush of honeycomb structure.
The honeycomb deforms plastically and crushes uniformly after exceeding its ultimate compressive strength at a constant stress level (vary for different core materials and density). This relation is depicted in figure 4.12
It results in easy anticipating of the absorption capacity of the honeycomb and its wide use as the energy absorption components.
In such a scenario, the core tends to crush a little when the compression peak is being removed. The crush strength of honeycomb decreases with increasing angle loading from the thickness.
Figure 4.12. Load-Deflection Curve
4.5.5. Shear Strength
Honeycomb sandwich due to its non-uniform structure is subjected to combination of shear strength and tension.
Shear strength depends upon parameters such as facing thickness, core thickness, facing material and loading conditions and it could be affected even by a very small component loaded parallel to the cell axis. The shear strength has a bigger value for thinner core and smaller value for thicker one. In case of very thin core the filleting of the core-to-skin adhesives may just strengthen the wall cell or fillet it entirely what has an impact on the shear strength of the honeycomb panel. The type of adhesive used in the structure also affects the shear properties as the filleting depth varies for different adhesives.
The another important factor for shear strength is the facing thickness as the skins carry shear loads in addition to what the core carries and can take on additional shear loads after the core has yielded.
5.1.Elements of Mechanical Behaviour of Composites
When discussing about mechanics of composites there is important to remember it is not a homogenous material but material consisting of two separates phases in a single lamina that builds laminate and entire structure.
As a consequence of it, there are two types of interactions in composites: micromechanical and macromechanical behaviour of composite material. The first one refers to interaction of constituents (fiber and matrix) and their impact on the single lamina in a laminate. The second concerns mechanical behaviour of composite material and structure ignoring all interactions at constituents level. Macromecanics behaviour is specified by stress, strain and the same mechanical properties as homogenous material. Micromechanics focuses on relation between composite properties and constituent properties.
Most composites are heterogeneous and anisotropic where properties determinates its orientation. The complexity of stress and strain behaviour is greater in case of anisotropic materials than in isotropic material. For instance, in case of isotropic material, a normal stress stands for normal strains and a shear stress for shear strains. Whereas, in an anisotropic composite, normal stress may concern both normal strains and shear strains, and a shear stress may concern both shear strains and normal strains.
An isotropic material exposed to a change of temperature undergoes expansion or constriction that is uniform in all directions whereas anisotropic composite under the same conditions demonstrates nonuniform expansion or constriction resulting in material distortion. These effects considerably affect characterization of composite behaviour along with the analytical mechanics of composites.
5.2. Fundamental mechanical theory
Stress is the internal resistance of a material to the distorting effects of an external force or load. These counter forces tend to return the atoms to their normal positions. The total resistance developed is equal to the external load. This resistance is known as stress.
The stress is force applied per unit area of a material expressed in newton per square meter unit [N/m2]. Where 1Pa = [N/m2].
where: is a stress
F is a force applied
A0 the cross-sectional area
The variant of this is tensile strength defined as the maximum stress a material can withstand without breaking, at an unchanged original cross-sectional area.
Strain is a ratio of increasing length to original length. There are two types of strain: positive (if fibers stretched) and negative ( if fibers compressed). Strain is measured by strain gages that covert mechanical motion into electronic signal. Strain is a non-dimensional characteristic, expressed in percentage.
Where is normal strain
L0 is the original length of a body
L is the final length of a body
Modulus of elasticity or Young's Modulus is expressed as the ratio of stress to strain in elastic region of the material. It indicates material's stiffness, or ability to resist bending.
Hooke's lawÂ ofÂ elasticity states that the extension of a spring is in direct proportion with the load applied to it. This rule applies as long as load does not exceed the material'sÂ elastic limit.
Materials obeys Hooke's law are known asÂ linear-elasticÂ . This law stats proportional strain to stress values, which is graphically illustrated as a linear curve.
In composites testing this law is useful for analysis of the tensile test when in the initial stage the ratio of stress and strain is a constant.
The test is usually performed on Universal Testing Machine and it is used to measure values of the residual strength, the energy absorption as well as the modulus of elasticity in bending Ef, Flexural strss Ïƒf, flexural strain Îµf and the flexural strss-strain response of the material.
Three point bend test involves placing a rectangular specimen horizontally on two supports where the load P is applied at the centre of the specimen in an x-y plane and it's distributed within its width (w=2c).
Fig Three Point Bending
The value of stress in N-N axis is zero, while in the y axis the stress represents tensile stress in the positive direction the compressive stress in negative direction. The resulting force is recorded.
For rectangular cross section the values of flexural stress in the outer fibers at midpoint and flexural strain in the outer surface are calculated with following equations:
The modulus of elasticity in bending is therefore:
The terms used in these expressions are:
P = load at an instantaneous point (N)
L = length of support span (mm)
b = width of test beam (mm)
d = depth of test beam (mm)
D = deflection at the centre of the beam (mm)
m = gradient of the initial straight line part of the load/deflection graph (N/mm)
(Charpy Impact Test)
The charpy test was invented by French scientist Georges Charpy in 1905 and approved in 1933.
The purpose of this test is to determinate the amount of energy that is absorbed by material prior to fracture after sudden, known amount of kinetic force is applied to specimen.
Results can be obtained quickly and cheaply however they are only comparative.
The charpy test involves breaking the specimen by a hammer on pendulum arm, under condition defined as standard. The important factor is temperature, as the resistance of material against shock decrease with decreasing temperature. The specimen is notched at its centre prior to testing and placed on two supporters, held firmly at each end. The hammer that is set into swinging motion and the striker impact the specimen that absorbs energy until it yields. The plastic deformation at the notch begins, however the specimen still absorbs the energy at the plastic zone at the notch. When further energy absorption is impossible, specimen fractures. The absorbed energy (in Joule) determinates the resistance of the material to shock loads.
The charpy impact machine
In tensile test the specimen is subjected to uniaxial tension until fractures. The test is used to determinate ultimate tensile strength, maximum elongation, reduction area as well as characteristics such as Young's modulus,Â Poisson's ratio,Â yield strength, andÂ strain-hardening.
In the test process a specimen is placed in the testing machine and a gradually increasing tensile load is applied along its longest axis until the sample failures. Acting of the tensile force causes elongation of the gauge section what is measured and registered by computer.
The specimen is a standardized sample cross-section, its shape remains a dog bone. It has two large shoulders needed to be firmly gripped in testing machine and a smaller gauge section where elongation and fracture occur.
A tensile specimen is a standardized sample cross-section. It has two shoulders and a gauge (section) in between. The shoulders are large so they can be readily gripped, whereas the gauge section has a smaller cross-section so that the deformation and failure can occur in this area
The equations 5.4-5.6. can be used to calculate composite properties, in tensile testing as well as following equations which are used are for the original cross-sectional area and reduction in area.
5.6. Drop-weight test
Drop-testing is commonly performed to specify the tolerance of an impact on the material.
In the testing process, the specimen is clamped rigidly to a heavy table that is guided along vertical rods or horizontal rails to have a single impact against the ground (or a vertical barrier). 
In drop test the rules for potential energy and the kinetic energy are used to measure the impact which specimen can withstand before failure.
The terms are:
EP = potential energy (J)
m = mass (kg)
g = acceleration due to gravity (m/s2)
Î”h = change in height (m)
v = final velocity (m/s)
u = initial velocity (m/s)
EK = kinetic energy (J)
Two rail shear test
Two-rail shear test method is used to measure in-plane shear data which is recorded by the strain gages.
The average shear stress along the specimen loading axes (x,y) can be described by simple equation:
Where L =specimen length along the y direction,
P = applied load along the y direction
t = specimen thickness.
Shear modulus, alsoÂ modulus of rigidity, denoted byÂ G can be calculated using the following equation:
(for + 45Â° or - 45Â° strain gage)
G = shear modulus [KPa]
Î” P/ Î” e = slope of the plot of load as a function of deformation within the
linear portion of the curve [KPa/m]
L = total length [mm]
t = thickness [mm
Two Rail Shear Test Method
In 2 rail test method a flat rectangular specimen with holes along opposite sides is bolted to steel parallel rails. The load is applied at the ends of the rails and distributed alongside each rail. The test is conducted according to the standard ASTM D4255/D4255M-01. The two-rail shear can be loaded in tension as well as in compression. On the specimen surface the strain gage is installed. It is three-element strain gage rosettes monitoring the resulting strains.
Figure 1 Assembly Rail Shear Apparatus
Significance and Use
The two-rail is a method in which in-plane shear properties for material specifications, research and development, and design.
The following are known as factors having an impact on shear stress: type and quality of the material, methods of material fabrication, orientation of filaments in laminate, machining of specimen, environment of testing, specimen alignment and gripping, speed of testing, time/length of testing, temperature, occurrence of voids, fiber to matrix ratio.
Two-rail technique is used to identify material properties such as:
In-plane shear stress and shear strain response
In-plane shear modulus of elasticity
Offset shear stress, and
Maximum in-plane shear stress.
Unfortunately, it is impossible to create pure and uniform shear stress condition to failure for every material. However there are test methods that can meet main engineering requirements and give fairly accurate results. In testing composite materials the two-rail method with off-axis load produces relatively small in-plane tensile load.
specimen fabrication- The following factors can affected properties of composite material: inadequate material production, creation of voids, incorrect machining, impropriate fiber alignment,
Buckling - Buckling of the specimen, particularly the thin once is the common problem while the load is applied in rail shear testing. In order to prevent it, the surface strains is measured on opposite sides of the specimens with three-element strain gage rosettes. It is misleading to interpret buckling as the maximum shear strength.
Delamination - Specimens with fiber orientation 45Â° are likely to undergo delamination when loaded in compression but it will not affect the strain gauge readings.
Gripping- the possible failure can occur due to impropriate gripping alongside the rails. The bolts can be too loosen or to tight fasten. There is also probability of inappropriate positioning of the holes as well as it's dimensions.
The assumption of two-rail methods is that pure shear occurs throughout the length of specimen gage section. Although, the gage section ends have zero shear stress as no traction and no constraints is applied there. A stress is distributed in the area from the ends to interior portions of the gage section.
The two-rail test method is valid for composites with length/width ratio less than 10 and Poisson's ratio (laminate to specimen edges) is less than unity.
If the Poisson's ratio of the laminates is high (45 Â° fiber orientation composites), the shear stress distribution is very irregular and leads to very low values of shear strength.
Hence two rail method requires relevant laminate orientation in order to obtain true values. The geometric structure affects composite during the test as follow:
The 0Â° fiber orientation contributes to creation of very high normal stress that is distributed perpendicularly to the filaments
The 90Â° fiber orientation causes premature failure of the filaments.
The aspect ratio influences considerably in-plane stress distribution
A Strain Gage
AÂ strain gaugeÂ is a device with an arrangement of two or more gage grids which determinates the normalÂ strainÂ along different directions on the surface of an object. The strain gage comprises of anÂ insulatingÂ flexible backing which supports a metallic foil pattern. The relevant adhesive (such as cyanoacrylate) is used to bond the gauge with the test part. Along object deformation, the foil is also deformed which leads to its electrical resistance to change, usually measured by a Wheatstone bridge.
There are three basic types of strain gage rosettes (each in a variety of forms):
â€¢ Tee: comprise two grids perpendicular to each other.
â€¢ Rectangular: consists of three grids, where first and second grids as well as second and third grids are positioned at angle of 45Â° to each other. Hence, the angle between first and third grids is 90Â°.
â€¢ Delta: built of three grids, all of which are located at angle of 60Â° to each other.
Figure Basic rosette type classify by grid orientation: a) tee; b) rectangular; c) delta.
The tee rosette should be used only when the principal strain directions are known in advance. Otherwise, the rectangular and delta rosette should be used as they are designed for applications where principle strains are unknown and can be installed regardless of its orientation. Geometric irregularities such as holes, ribs etc can affect the principle directions, hence in this case additional precautions may be taken.
A strain gauge operates based on the physical property ofÂ electrical conductanceÂ and its dependence on the conductor's geometry.
When anÂ electrical conductorÂ is stretched and reaches the limits of itsÂ elasticityÂ in the manner that it does not break or permanently deform, it will become narrower and longer, changes that increase its electrical resistance end-to-end.
From the other hand, when a conductor is compressed, but not buckle, it will broaden and shorten, changes that decrease its electrical resistance end-to-end.
Electrical resistanceÂ of the strain gauge is recorded and on that that based applied stressed may be calculated. Stain gage measures only deformation on the limited area of an object and can be manufactured small enough to establish stress principles within "finite elements"
The voltage of 5V or 12V is applied to applied to input leads of the gauge network, and a voltage reading is taken from the output leads millivolts.
The most common are foil strain gauges, they are attached to test part with a special glue. There are many types of glue depending on time scale of measurements. Eg. for short term installations (up to few weeks) cyanoacrylic glue can be used, but to bond gauges with on object for longer period the epoxy glue is required.
The surface preparation is very important. It must be smoothed, neutralized, deoild and glued immediately after preparing in order to avoid its contaminating. Otherwise obtained results may be unreliable and generate unforeseen errors.
The nature of investigation of fibre carbon laminate and honeycomb sandwich demands dividing apparatus into two categories: manufacturing and testing.
Apparatuses used for manufacturing:
Mold - the glass plate used as a template.
Carbon fiber sheet - a raw material of carbon fiber.
Nomex - a material used as a core in a honeycomb structure.
Bleeder (perforated plastic sheet) - a material used to absorb the excess resin from the specimen during curing.
Breather (felt sheet) - a material that prevents creation of the voids from the air trapped between the layers.
Vacuum bag (pulyethane bag) - the material that enables to maintain the vacuum during curing
Release agent (wax) - the substance covering the mold aiming at easy and safe removing the cured specimen.
PCT-2A Cellophane Tape
M-Bond 200 Catalyst
M-bond 200 adhesive
Apparatuses used for testing:
The Universal Tensile Test Machine
The Universal Drop Test Machine
The Universal Charpy Test Machine
The Tensile-Compression Testing Machine
PC and printer
Laptop operating the software LabView 7 Express made by National Instruments (Texas).
Memory Card: DAC CAD A1-16E-4
The signal contribution unit SC 2043 stroke/SG
Strain gage instrument
Specimens: 3 dog-bone, specimen with notch, square specimen with four drilled holes in the corners, honey comb rectangular and square specimens, two rectangular specimens with six drilled holes
7. Manufacture of composite
7.1. Manufacturing of specimens for tensile/ impact/ 3-poit bend and charpy test purposes.
The student begun to manufacture the carbon fiber composite components by measure the carbon fiber sheet and marking on it five squares of dimentions 40cmx40cm with fiber orientation of 0Â°, 45Â°, 90Â°, 45Â°, 0Â°. It was decided that the large square composite laminate(40cmx40cm) would be manufactured first and then trim to dimensions required for testing purposes. A few difficulties relating to measuring and cutting carbon fiber sheets were encountered. The problem represented not "stable" structure of the sheet resulting in movement of single fibers orientation by touching or delicate pulling what required student's carefulness and precision. In addition, cutting material caused unpleasant dust which inhaling wasn't recommended.
For manufacture composites in wet lay-up method it was crucial to prepare all equipment prior to the process.
As the first, a bleeder, the breather and a vacuum bag were cut respectively to dimensions 45x45cm, 50x50cm and 65x65cm. On the breather the mark was made and then using the hole-punch and hammer the hole was cut out for vacuum port purposes. In order to ensure the smooth surface of the component and eliminate possible contaminations, the glass mold was cleaned appropriately using the sandpaper, a brush and acetone. The mylar tape were used as a sealant and formed to a square shape of dimensions of approximately 60x60cm. The mold were coated with release agent to ensure smooth separation and minimalize possible mechanical damage after component would cure.
The matrix preparation had to take in very short time before the start of the lay-up due to its fast hardening properties. The epoxy resin and hardener were mixed with a 3:1 (120g of epoxy and 40g of hardener) ratio in the paper cup with noticeable heat emission. There was a necessity of wearing a latex gloves by the student in order to prevent the skin from chemical damage.
Once all equipment was prepared, the student started placing the carbon fiber sheets in mentioned above orientation 0Â°, 45Â°, 90Â°, 45Â°, 0Â°. Each fabric layers was covered by the epoxy with a use of brush that enabled its even spreading. The bleeder and breather bag were positioned on the lay-up component and the vacuum port was placed through pre punched hole in the vacuum bag. The special care was took while bag was being put across the specimen as any trapped air would lead to void creation. Then, the vacuum bag was stuck to the seal tape and checked if there is any tears. The vacuum port was connected to the hose of the vacuum pomp, turned on and left for 5 hours to cure.
The same activities were repeated for manufacture of honeycomb sandwich with a difference that Nomex core were used between two laminates.
The cured specimen were then cut to dimensions required for testing purposes. It will be discussed in the next chapter.
Manufacturing of composite for Two Rail Shear Test purposes
The specimen were manufactured in hand lay-up method described in section 7.1. The pictures for manufacturing process can be found in appendix G.
After curing time, student cut out six rectangular specimens of dimensions 152x76mm.
Then six holes in each specimen were drilled with driller in figure 1, appendix J.
The exact dimensions of specimen are depicted in figureâ€¦..
Figure Two-Rail Shear Specimen
Strain Gage Instalation
First, student polished the surface with the sandpaper in order to ensure the surface is smooth and without any irregularity. Then respectively conditioner and neutralizer were applied to the top of specimen with a cotton tissues and by circular movements. Once the surface was ready, student started preparing the strain gage that would fit the centre of the specimen.
Student removed the gage with tweezers from plastic envelope and placed it on clean glass plane. As the gage needed to be soldered with terminals, they had to be prepared as well. Student removed long unit of terminals and cut it in pairs with pincers taking care they didn't undergo any damage or contamination. Then the PCT-2A cellophane tape was used to keep terminals at the right position and at the distance about 1.6mm from the gage. Student drew a centre lines with a pencil on the specimen to assure the gage would be exactly in the centre of specimen. Then the M-Bond 200 Catalyst was applied on the carbon fiber specimen. Student prepared a cotton tissues as it was needed for pressing the gage to the specimen for about one minute (once the adhesive is applied the curing time is very fast). The M-bond 200 adhesive was applied at the centre of the gage as well as at terminal bottoms. Then student placed the gage at the crossing point of centre lines and with mentioned above cotton tissues made sure the gage with terminals were firmly attached to the specimen. Student allowed about one week the adhesive to dry properly. Then the cellophane tape, which kept terminals in right position before bonding was removed. The gage and terminals were soldered with each other and got ready for next stage - wiring.
Student cut eight cables, each cable was about one meter long and consisted of four thinner cables in different colours: white, red, black and green. As only three of them were needed student removed red one and slightly separated the remaining three from each other at both ends. About 8 mm of plastic coating was removed in order to enable its further soldering what can be seen in appendix J, figures 12, 13. Student coated the ends of wires with solder and then soldered them at one end with the port connector in relevant sequence and at the other end with gage.
Ready for testing, specimen was connected via the port connector with SC 2043 stroke/SG signal contribution unit which was directly linked with the computer shown in figure 19, appendix J.
The strain gages were installed on the carbon fiber (1) and carbon fiber (2) specimen with orientation as is shown on figure below:
Figure strain rosette orientation in carbon fiber specimens respectively (1) and (2).
As two specimen of fiber carbon were manufactured and used in testing stage, in further description carbon fiber specimen with channels 0-2 will be described as carbon fiber (1), and carbon fiber specimen with channels 2-5 as carbon fiber (2).
Figure Schematic of nomenclature of carbon fiber specimens used in Two Rail Shear Testing.
The specimens which undergone testing were made of 5 ply continuous carbon fiber with orientation 0Â°, 45Â°, 90Â°, 45Â°, 0Â°. First of all, the Universal Tensile Test Machine as well as PC were switched on, running the program for graphical analysis. The machine was set at the speed rate of 12.5mm/min.
Using a Vernier Calliper the specimen were measured and the values were insert into program.
The specimen were positioned at the testing place and the tensile test machine began testing after switching on the control panel. Two samples of the specimen made of the same laminate were tested. Initially, there was a concern about proper calibration of the scale as the forces and deflection axes could have too small or too value but the graphs and results were correct.
During the test student stayed reasonably away as there was a possibility that flying debris could injure the eyes. The same procedure were repeated for testing honeycomb specimens.
The tested specimens were the same laminates as it previously described.
After measuring specimens with Vernier Calliper the first specimen were placed into jaws and the machine calibrated adequately in order to provide the specimen with proper grip. After, placing the first sample the uncertainty about correct grip arose. During the test it was found that the first specimen fractured very quickly. It was decided to repeat the test using the same settings but with operator that would hold the specimen in the right position. The second testing was successful and allowed to obtain the proper results. The same activity was conducted while testing a honeycomb.
The drop test were carried on using the same samples. After measure the specimen it was placed on the of the jig consisting of lower and upper. The specimen was attached to the jig base with four screws in order to hold it in stable position, therefore all samples had to have drilled matching holes.
The bar that was dropped onto the specimen was placed manually into the rig at specific height, then clicked into position to hold. After, taking all measurements and firmly placing the testing sample, the bar was released and fall down at the specimen. The damage magnitude of the specimen specified the capacity for energy absorbed of the bar dropped from specific height. The honeycomb specimen was tested using the same procedure.
In this test only the one specimen was tested made of single laminate with skipping the honeycomb sandwich sample. The sample was placed in the notch and pendulum axe launched into swinging motion. After sample fractured the result was read.
Two Rail Shear Test
Prior to testing specimen with installed strain gauge, one of spear specimens were destructive tested in order to investigate load needed to break the sample (appendix J, figures 16-18). As the specimen broke under the load 22.75KN, student knew how to calibrate the machine properly.
The specimen with strain gauge were connected to the computer operating the software LabView 7 Express made by National Instruments (Texas). There were also used: The signal contribution unit SC 2043 stroke/SG and memory card DAC CAD A1-16E-4. Their pictures can be found in Appendix J, figures 19 and 20.
After checking if the circuit works correctly, student placed the specimen into a jig and fasten bolts. The jig with a specimen was placed into a tension-compression testing machine, and machine calibrated. Student were applying the load gradually by turning the wheel of testing machine anti-clockwise. The load was increasing by 0.5 KN after each time student had to take the reading of deflection and strain of three channels displayed on the computer screen. The illustration of channels layout is depicted in appendix L, figure 4. It was important to take the reading quickly as soon as the load was applied in order to obtain accurate results. It was connected to changing value of strain under load being increased. The figures for channels appearing on the computer screen were changing values very fast as the material was deforming, what could affect the accuracy of taking the proper value.
During testing the first specimen marked as carbon fiber (1), the cracking sound at the load of 19 KN. The specimen fractured after applying the load of 24.5 KN and the load value on Tension-Compression Testing Machine was immediately dropped to 0 KN.
Carbon fiber (2) specimen started to break at the load of 18KN as then the characteristic cracking sound was observed. The load of 23.5 KN caused fracture of the specimen, however it proceeded in different manner than in specimen (1). The load on testing machine could have been still applied, but it was gradually decreasing.
The results obtained from testing the single laminate specimen with fiber orientation 0Â°, 45Â°, 90Â°, 45Â°, 0Â° are shown in the graph 2-3 Appendix C
On the graphs are shown he values for flexural strain at rapture which is expressed as a percentage of the gauge length, and three point bend stress (flexural stress).
The tables and graphs contain all important information such as values of stress and strain.
The specimen dimensions as well as all data were entered into a spreadsheet which along with adequate equations discussed in chapter 5 were used to produce a graph of stress/strain curve and a chart of energy absorbed up to ultimate tensile stress graphs 1 in Appendix E
As it is seen on graph 1in Appendix E the carbon fibre 5 ply specimen fractures at higher stress than honeycomb sandwich specimen. The values of stress at rupture are respectively 3.547 MPa and 3.047 MPa. The strain value is much higher for honeycomb sandwich than for carbon fiber 5 ply and it accounts for 19% and 4.5% respectively.
It could be noticed that the single lamina has much higher Young's Modulus 0.79GPa than sandwich structure 0.16 GPa.